Explain how a change in pH above and below the optimum affects the action of an enzyme.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Enzymes are biological catalysts that control chemical reactions in living cells. Each enzyme has an active site with a specific shape that is complementary to its substrate.
Model answer (5 marks)
Enzymes have an optimum pH at which they work best.
If the pH is higher or lower than this optimum, the charge on ionisable groups in the enzyme changes.
This alters the shape of the active site, so the substrate can no longer fit or the site is no longer complementary.
Consequently fewer or no enzyme–substrate complexes are formed, and the reaction rate falls.
If the pH change is severe, the enzyme becomes denatured – a permanent change in the active‑site shape that abolishes activity.
If the pH is higher or lower than this optimum, the charge on ionisable groups in the enzyme changes.
This alters the shape of the active site, so the substrate can no longer fit or the site is no longer complementary.
Consequently fewer or no enzyme–substrate complexes are formed, and the reaction rate falls.
If the pH change is severe, the enzyme becomes denatured – a permanent change in the active‑site shape that abolishes activity.
Examiner tips
- State the optimum pH concept first. Show the effect of pH on active‑site shape. Explain loss of substrate binding and reduced rate. Mention denaturation for extreme pH.
Common mistakes
- Assuming pH only affects rate, not structure. Forgetting to mention denaturation. Using vague terms like "changes" without linking to charge/shape.
Mark scheme (5 marks)
- Enzymes have an optimum pH at which they work best / fastest
- As pH moves away from the optimum, the shape of the active site begins to change / is altered
- This means the substrate can no longer fit into the active site / the active site is no longer complementary to the substrate
- Fewer / no enzyme-substrate complexes can form
- If the pH change is severe / extreme, the enzyme becomes denatured — this is a permanent change in the shape of the active site
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Biological reactions and enzymes questions
- Catalase is an enzyme found in liver cells that breaks down hydrogen peroxide in…
- Explain how increasing temperature affects the rate of an enzyme-controlled reac…
- Proteases and lipases are enzymes found in the digestive system. Explain how the…
- Amylase is an enzyme that breaks down starch into simple sugars. Explain how the…